450 The Blight in the Pear Tree ; 



niim, which has probabl)'- been bm little injured ; the leaf 

 puts out, and no outward sign of disase appears; nor will 

 it appear until the leaf prepares the downward current. 

 May, June and July are the months when the growth is 

 most rapid, and when the tree requires the most elaborate 

 sap ; and in these months the blight is fully developed. 

 When the descending fluid reaches the point where, m the 

 previous fall, a total obstruction had taken place, it is as 

 effectually stopped as if the branch were girdled. For the 

 sap which had lodged there would, by the winds and sun, 

 be entirely dried. This would not be the case if the sap 

 was good and the vitality of the wood unimpaired ; but 

 were the sap and vessels are both diseased, the sun affects 

 the branch on the tree just as it would if severed and lying 

 on the ground. There will, therefore, be found on the tree, 

 branches with spots Avhere the bark is dead and shrunk 

 away below the level of the surrounding bark ; and at 

 these points the current downward is wholly stopped. 

 Only the outward part, however, is dead, while the albur- 

 num^ or sap wood, is but partially injured. Through the 

 alburnum, then, the sap from the roots passes up, enters 

 the leaf, and men are astonished to see a branch, seemingly 

 dead in the middle, growing thriftily at its extremity. No 

 insect-theory can account for this case : yet it is perfectly 

 plain and simple when we consider that there are two cur- 

 rents of sap, one of which may be destroyed, and the other, 

 for a limited time, go on. The blight, under this aspect, is 

 nothing but ringing or decortication, effected by diseased 

 sap, destroying the parts in which it lodges, and then itself 

 drying up. The branch will grow, fruit will set, and fre- 

 quently become larger and finer flavored than usual. 



But in a second class of cases, the downward current 

 comes to a point where the diseased sap had affected only 

 a partial lodgment. The vitality of the neighboring parts 

 was preserved, and the diseased fluids have been undried 

 by wind or sun, and remain more or less inspissated. The 

 descending current meets and takes up more or less of this 

 diseased matter, according to the particular condition of the 

 sap. Wherever the elaborated sap passes, after touching 

 this diseased region, it will carry its poison along with it, 

 down the trunk, and. by the lateral vessels, in toward the 

 pith. We may suppose that a violence which would des- 

 troy the health of the outer parts, would, to some degree, 



